Unit of energy measurement

Definition of Lux

Lux measures illuminance in terms of the amount of light that falls on a surface per unit area.

How is lux measured?

One lux is equal to the illumination of a one meter square surface that is one meter away from a light source.

How does this differ from foot-candle?

Lux is a metric measurement, while foot-candles belong to the imperial measurement system. They’re two sides of the same coin.

To convert foot-candles to lux, you would multiply the foot-candle value by 10.764.

Conversely, to convert lux to foot-candles, you would divide the lux value by 10.764.

Why is Lux important?

Most lighting fixtures list a lumen figure and while this is useful, it can fail to inform you how effective the light is in a real world setting.

Factors such as electrical efficiency, clarity of lens, size and geometry of lamp reflectors, and the angle of the light can greatly reduce or alter the lumens of a light source.

This is why you need the real world measurement of lux to gauge a lamp’s performance, especially those that need to light a wide area or a significant distance.

In practice, a flood light like the Rigid 321113 D-XL Series Pro 4" Flood producing 7128 lumens gives you:

  • 147 lux at 10 m — equivalent to an overcast day
  • 1 lux at 121 m — equivalent to full moon on a clear night
  • 0.25 lux at 242 m — minimum for escape lighting and emergency exit signs

It tells you exactly how the distance and amount of light the lamp will give you. Maximum lux is also useful in comparing one light fixture to another.

Example of Lux in a Sentence

"The lux meter indicated that the workspace was brightly lit with an illuminance level of 1000 lux."

Related Terms for Lux